1. Field of the Invention
The present invention generally relates to disk drive data storage devices and, more particularly, to methods and apparatus for temperature-adaptive open-loop control of active latch.
2. Description of the Related Art
Disk drives are widely used in computers and data processing systems for storing information in digital form. Conventional disk drives include a head stack assembly, one or more data storage disks and a spindle motor that rotates the storage disks. The head stack assembly includes an actuator motor and an actuator arm assembly that includes read/write heads mounted to flexure alms. The actuator motor can rotate the flexure arms and read/write heads about a pivot bearing relative to the storage disks.
The read/write heads are configured to fly upon air bearings in very close proximity to the rotating storage disks. Unfortunately, contact between the heads and the storage disks can result in damage to the storage disks and the actuator arm assembly.
In some disk drives, the actuator motor positions the heads over a landing zone on the disks as power is removed from the spindle motor. The landing zone can be a ramp that is positioned near each of the storage disks. Alternately, the landing zone can be a textured, non-data region of each of the storage disks.
However, even when the head is positioned safely in the landing zone, a sufficient force or shock to the disk drive may cause the heads to move from the landing zone onto data storage surfaces of the storage disks. Conventional disk drives attempt to address this problem with a latch that inhibits movement of the actuator arm assembly, and thus the head, relative to the storage disks during non-rotation of the storage disks.
One type of convention latch is a bi-stable latch that can be moved between a closed position and an open position. In the closed position, it latches the actuator arm assembly to inhibit movement of the head from the landing zone. In the open position it allows the head to be freely moved from/to the landing zone. A bi-stable latch typically includes a coil and a bias device such as, but not limited to a permanent magnet. The bias device can maintain the latch in the closed position while the disk drive is turned off. To open the latch, a sufficient current is conducted through the coil to overcome the magnetic force of the permanent magnet or bias device to cause the latch to move to the open position. The latch is closed by removing the coil current, which allows the permanent magnet or bias device to return the latch to the closed position. The coil current is typically generated through an open-loop control circuit without latch position feedback because a sensor for sensing latch position can be costly and add to the complexity of the latch. Accordingly, in operation the latch is alternately switched between the open position where it rests on the coil or an open position limit and the closed position where it rests on the bias device or a closed position limit.
When the latch switches between the open and closed positions, the alternating contact of the latch with the coil and bias device may cause undesirable noise to be generated from the disk drive. The latch switching may also cause wear to the latch and/or actuator arm assembly, which may generate loose material within the disk drive that can damage the heads, storage surface of the disks, and/or other components of the disk drive.
The open loop latch operation (open/close) current or voltage profile can be shaped to achieve several benefits, such as quiet operation, less wear and tear, reliability and power conservation. However, the performance of the open-loop current/voltage is subjected to the large variations of temperature operation range for Hard Disk Drives (HDD).
The current applied is very often provided via Pulse Width Modulation mode (PWM). At lower temperatures, the coil resistance is smaller and the effective current in the coil with the same PWM duty cycle and frequency is larger. On the other hand, when the temperature is higher, the effective current in the coil will be smaller due to the higher resistance of the coil. As a result, the magnetic field generated at a lower temperature is stronger than that generated at a higher temperature, thus resulting in different performance of the latch.
Previously, the performance of the latch was compromised to cover a wide operation temperature range. For example, to make sure latch will open reliably, the open and hold current/voltage had to be designed based on the worst case scenario—based on the highest coil resistance and lowest power supply voltage. As a result of this, a number of undesired effects such as power waste, extra latch wear and tear, and noisy operation can be generated.